CoPilot:
Starter vs. Water Pump Replacement
Since both tasks are quite involved, it's generally a good idea to start with the starter replacement first. This is because the starter is located in a more accessible area compared to the water pump, which requires removing many components like the timing belt, pulleys, and possibly the intake manifold. By tackling the starter first, you'll avoid having to reassemble and then disassemble parts again.
Cluster Gauge Map
As for the cluster gauge map, I couldn't find a specific map for the 1991 Lexus LS400, but you might be able to find it in a repair manual or by contacting a Lexus dealership for detailed schematics.